The present invention relates generally to systems for controlling the parameters of a physical process, such as the gauge of a metal strip being reduced in thickness in a rolling mill, and particularly to a method that substantially eliminates the effect of transport delay of the material in taking corrective action when a disturbance in a nominal value of the material, such as strip gauge, is detected.
Automatic thickness or gauge control systems employed in rolling mills typically use on-line thickness measurement devices located downstream from the exit side of the mill that feed back information on material thickness for effecting corrective control action. The thickness measurement is not generally available at the time the strip leaves the bite of the work rolls of the mill, the measurement being delayed by the amount of time it takes for the strip to travel from the roll bite to the measuring device. In process control terms, the sheet thickness at the exit of the roll bite represents an unmeasurable state or variable.
Hence, one consequence of material transport delay in feedback controllers is that disturbances in the manufacturing process cannot be detected when they occur, and the rate of corrective action is limited because system response to a control action is not known until sometime in the future.
Among the techniques used to deal with linear control systems involving transport delay are the Smith Linear Predictor and "state observers". The former involves the use of a transfer function model GM(s) of the process in a control loop to control process actions. This is shown in FIG. 1 of the present drawings. When a disturbance is detected (still after-the-fact detection), an electrical controller receiving information from the transfer function model can quickly perform the required correction. This is usually fully accomplished before an actual measurement of the manufacturing process becomes available, if one is taken at all. In FIG. 1, the Smith Linear Predictor includes an outer loop by which an actual measured value, when it becomes available, is used to correct for errors that might exist in the transfer function model.
State observers, on the other hand, employ process models to estimate unmeasurable states, such as the gauge of material immediately leaving roll bites, as a function of a measurable value, such as the size of the roll gap of the mill rolling the material. Control action is then based on such an estimate. A recent (Mar. 1990) patent on a state observer employed in rolling mills is U.S. Pat. No. 4,907,434 to Hoshino et al (Sumitomo Light Metals). In addition, a paper on this subject by Hoshino et al was presented at the IFAC World Congress in Munich in Jul. 1987 and published in a journal entitled Sumitomo Light Metals Technical Report, Jul. 1987, under the title of "Observer-Based Multivariable Control of the Aluminum Cold Tandem Mill." As will be noted from both the patent and paper, state observer methods typically require detailed process models and can lead to complex sets of equations that must be solved in performing the control function.
As with the Smith Linear Predictor, state estimates of the manufacturing process are correctable when an actual process measurement becomes available.